Electronic – Why the difference between the RSSI and Rx Level measurements is so big

A splitter definitely won't work. It's a device that accepts an input signal and delivers multiple output signals.

I think that you need a directional coupler. It has an input, an output, a coupled port, and a load. It's a device that operates on an input so that two output signals are available, on the output port (larger signal) and the coupled port (smaller signal). However, when the input is applied to the opposite port of a terminated coupler, only one output signal is produced. MCL makes suitable devices.

This is a harder problem than I think you realize. The basic Loc8tor system works primarily by virtue of a directional antenna in the receiver, and it doesn't really give you the position of the tag directly, it just indicates what direction it is from the location of the receiver, along with a very rough estimate of how far away it might be.

If you want to create a mesh of automatic receivers, each of these receivers will need to do direction finding, either mechanically by physically spinning its antenna in a circle, or by using multiple antennas and electrically "spinning" the reception pattern. Neither method is going to be simple to implement or particularly low-power.

It is only by combining direction information from two or more receivers (plus knowing exactly where these receivers themselves are located) will you be able to derive an absolute position for the tag.